Manufacturing Process Study and Certification Dr. M.Elbert, R.Howe

Digital Equipment Corporation

INTRODUCTION The importance of product quality and reliability, time to market, cost and customer satisfaction continue to grow as the competitive battlefield expands.

Manufacturing processes have the most important role in producing products that meet cus- tomer expectations and required level of product quality/reliability. Why '? Average industry data indicate that manufacturing process problems represent approximately 60 % of all product problems, and material and design product problems represent 30 % and 10 % respectively.

This paper presents our approach in the area of manufacturing process study and certification program aimed to assure that the manufacturing process consistently/repeatedly produces prod- ucts that meet required quality and reliability goals.

This paper presents principles, plans, processes, structure, guideline, performance measure- ments and criteria necessary to achieve and ma.intain manufacturing process requirements and certification. Utilization of FMEA (and RPN), Process Audit, Process Perfomance / Capabil- ity Study (Cpk, PPM - defect rate, DPMO), Statistical Process Control (SPC, control charts) are discussed.

The key to an effective manufacturing process is to have measurements of process perfom- ance/capability that reflect customer expectations, and quantify the results of work performed by the process.

Such aspects as the differences between process performance and process capability, different conventional statistical theory tools to define acceptance tests approaches and sample sizes are discussed in the paper.

OBJECTIVES OF PROCESS CERTIFICATION

The overall goal of the process certification program is on assuring that manufacturing opera- tions consistently produce products that meet required quality and reliability goals.

Process certification provides the organization with a stable, measurable manufactukg process that meets customer expectations. A systematic process certification is a way to move from an appraisal-type philosophy into a preventive-type philosophy.

The ultimate long-range objective for process certification is provision (and demonstration) that the process is capable of producing zero process defects at sustained line output levels for a period of time. It implies the need for never-ending improvement. The Cpk level > 2.5 can be considered as a goal for provision of zero defect.

DEFINITIONS The general terms and definitions used in this paper are presented below.

Manufacturing Process is unique combination of machine, tools, methods, materials, and people used to manufacture or fabricate a product. This combination can apply to the entire process or to the individual operation. For the various manufacturing groups (Modules, Sys- tems, Software) the process can be defined as combination of operations. For example, for module manufacturing the process is defined as each workstation within the process.

Process Certification is assuring of (1) the required organizational and functional structure, and (2) required performance of the complete manufacturing process.

(1) Organizational and Functional Structure of Quality System. A certification process should demonstrate that all the necessary responsibilities, training, documentation, measurements, controls, etc. are in place to ensure that the manufacturing process can produce the required level of product quality.

( 2 ) The required level of performance should be measured and demonstrated. Also, the process shall demonstrate that it can repeatedly produce the required level of product quality.

Certification applies to the complete process and to a single operation. For the complete process to be certified, each operation in the process should be certified.

Qualification Process is the process defined to ensure that requirements specified for a prod- uct are met. The qualification process consists of a sequence of environmental, mechanical, electrical, functional, software, regulatory, safety and other tests which are conducted in accor- dance with the requirements of Qualification Strategy.

The Process Control is defined as a system for measuring and analyzing of the manufacturing processes. The process control system contains three major elements - data system, process perfomance/capability measurements/quantification and a feedback loop through which it compares with requirements, and act on difference.

Statistical Process Control (SPC) is the application of statistical techniques for measuring and analyzing of the processes (control). Statistical process control verifies the stability of the process and homogeneity of the product. Process PerformanceKapability is the capacity or ability to reproduce product characteristics or degree of quality attribute.

Process performance and capability are measurable properties of the process.

In general, process capability is expressed in terms of the Cp and Cpk measures where Cp is a measure of product uniformity and Cpk is a measure of product uniformity and target centered- ness. Performance Index Cpk is estimated by:

I x - L S L x - U S L I

1 I 3s 3s I

Cpk = fie { __------_____ _____________

where LSL and USL are the lower and upper specification limits respectively, and s is stan- dard deviation.

Process performance is not the same as process capability. The difference involves the assump- tiodstate of statistical control. If a process is in statistical control, than the measure of process performance results in determining (inherent) process capability.

It is important to distinguish between the process in a state of statistical control and process that is meeting specification. A state of statistical control does not necessarily mean that the product from the process conforms to requirements or specifications. On another hand, the process can meet the product tolerance and requirements even with a nonSPC status.

Fig. 1: Process Certification “Whv” Process

Provfdeorganlzatlor &functlonal structure

ldentlfyparamateffi crltical to process I RPN Develop mrrectlve actlons Fall safe operatlons

VaildateSPC rc YES

P ~ c a p a b i l l ~ S p c SRKty

Defineprocess

ValldateFMEA capability

Achievecertiflcatlon Provldedefect alterla

Maintain defect aiteria

Achleve 0 defect

f

All U 0 complete wlth approvals

elements

ldentlfled

RPN Pareto for all

Controlsystem

SPC demonstrated All Cpk measured All DPMO and defect rate measured ConlJoi charwtlistograms developed

Odefect demonstratlon on =we

Sustain output level

Sustain output level pproach longterm

Change requirements - DPMO defect rate - benchmarklng

Metrics

Provislon of all requirements (1 ooO/)

RPNPareto NocritldFMEh elements with (>25 RPN)

Ope~tlOnS Percent fail safe

Percent elements with Cpk > 20 All Cpk > 1.33 All PPM < 63

Demonstrate required DPMO &defect rate

Percent processes cemed by production line

Demonstrate requlred DPMO on mminuous bases

Cpk > 2.5 PPB- 1.0

CERTIFICATION STRUCTURE / GUIDELINE The flow diagram of the process certification process is presented on Fig. 1.

Process certification is an iterative process. Results of activities in any phase may require cor- rective actions, feedback loops and improvements of the previously developed phase.

The entire process ceftification structure is presented in the Process Certification Road Map (Table 1). The structure contains three fundamental components - Vertical structure, Horizontal structure, and Measurements which are used to measure the result and fulfillment Qf certifica- tion activities in each particular phase.

Table 1

Process Certification Road Ma

CFRTlF lCATlON SYSTFM APPl CATION fWHFN 8 WHFRF") MEASUREMENT - flAQuS2 STAGE 1 STAGE 2 SORWARE Each Operation Complete Process

Method Material Operator Test 8 Process EquipmenVCombination

PROCESS AUDIT > IS0 9000 & Speufic Check Lists

> Responsibility and Management > Measureable Goals and Customer

Expectations z Documentation > Row Diagram > Control (SPC), Data System and feedback

> Corrective actions > Training of production personnel1 > Operational requirements (Envrionments,

FMEA, Producttraceability, etc.) > Output aaxptabi l i

> Identify potential failure modes and cMuses, RPN Identify corrective actions (failsave technique)

> Define critical product parameters and process variables

> Idenbfyfimprove control

> Data collection system

z Process perfomancekapabilii study z Pareto analysis > Feedback adjustments > Design of experiment

systems

FMEA

PROCESS CONTROL

Statistical process control

PROCESS ACCEPTABILITY PROCESS

PROCESS IMPROVEMENT MONITORING/MAINTENANCE

PROVISION OF ALL ITEMS IN CHECKLISTS

RPN, LIST OF CORRECTIVE ACTIONS,

CRITICAL FACTORS

SPC Cpk, DEFECT RATE, DPMO

FMEA VAUDATlON

ZERO DEFECT for SAMPLE Cpk, DEFECT RATE, DPMO

on Continuouse Basis Cpk. DPMO BENCHMARKING

Vertical Structure. The vertical structure defines specific certification subprocesses/ activi- ties / phases which represent "whats"of certification. The vertical structure contains the follow- ing major activities: Process Audit, Failure Mode and Effect Analysis (FMEA), Process Control, Process Performance (Capability) and Statistical Process Control (SPC) Study, Proc- ess Acceptability, Process Monitoring and Maintenance, and Process Improvement.

Horizontal Structure. The horizontal structure defines implementation / application of certi- fication subprocesses / activities which represent "whens and wheres" of certification.

The complete process or separate operation can be represented as any specific combination of (1) Machines, methods, material and people, and (2) DZferent manufacturing subprocesses (Modules, System, Software).

The intersection of the vertical and horizontal structures represents combinations/blocks of dif- ferent certification subprocesses. The operator cerdfication, equipment calibration and main- tenance, provision of tracebility of the product are examples of specific certification subproc- esses.

Measurements (see Fig. 1, Table 1) defines metrics which are used to measure the result / ful- fillment of certification activities in each particular phase/subprocess, and which are used to exit this phase and enter the following phase of certification activities.

CERTIFICATION ACTIVITIES / PHASES - "WHATS"

PROCESS AUDIT

The objective of the audit is to assure that the required organizational and functional structure of the manufacturing process is provided. The systematic process audit is recommended to be performed twice per year. The audit should be repeated if a major change occurs in the process.

Process certification in this stage is achieved through compliance with the audit checklists. The process audit consists of two phases: (1) compliance with IS0 9000, and (2) compliance with specific company requirements. Two Process Certification Checklists are recommended: IS0 9000 and Detailed /Specific Checklist.

The process audit is organized into the following categories:

a. Responsibility and management b. Measurable goals and customer expectations c. Documentation d. Flow Diagram e. Training of production personnel f. Equipment and Tools g. Operational Requirements (including proper environment, FMEA, process tracebility, etc.) h. Control , data system, feedback i. Process SPC and Capability j. Corrective actions k. Achieving Certification.

Table 2

DETAILED/SPECIFIC CHECKLIST - EXAMPLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

mZE: SYSTFCMPROCESS CERTIFICATION PAGE ............................................................................. ELEMENT : STAGE 2 Description POOR MARG. -----

PACK Are Ioose-piccc locations spccified?

mZ~aGzniz&m&r----- E s B f l T s t match thc rack I-$-- - H F t K i ~ ~ ~ r X i Z i Z n g u n i i eliminated (arc parts picked !o a sinde box?)?

0 0 0 0

0 0 0 0 0 0 0

0 0 0 0

0 0 0

0

0

0 0 0 0 0 0

0 0 0 0

0 0 0 0 0 0 0

0 0 0 0

0 0 0

0

CERTIF.

0

0 0 0 0 0 0

0 0 0 0

0 0 0 0 0 0 0

0 0 0 0

0 0 0

0 . - - - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Detailed / Specific Checklist is used to provide and con€om specific elements and require- ments of the complete process and each operation. Use of the checklists should emphasize the

identification of elements needing improvement. Specific checklists control many specific de- tails of operations. Examples of Detailed / Specific checklists are presented in Table 2.

The documented evidence of compliance through systemic audits for all items should be pro- vided and satisfactory before the CeaLfication of this phase is granted. No any score/rating is recommended.

FAILURE MODE AND EFFECT ANALYSIS ( FMEA). Next phase of certification process is Failure Mode and Effect Analysis. The major objective of M A in this phase of process certification is identlfy potential critical processes/operations and product elements required improvement / corrective actions.

FMEA classifies and rank each potential failure mode according with risk. Risk Priority Num- ber (RPN) is used as a quantitative measure of a combined effect of Severity (l), Priority of occurrence (Z), and Likelihood of the problem Detection (3) .

RI" = (1) x (2) x (3) RPN order is (1-1000). RPN critical value/threshold is usually 100-125 (this value is condi- tional and depends upon an approach taken to measure severity, occurrence and detection).

Corrective actions are developed and prioritized based on FU". Problem solving approaches are used in this phase to develop corrective actions. Wide range of corrective actions shall be applied. Special attention shall be put on utilization of fail-safe techniques that do not allow de- fects to be passed to the next operatiodwork station. A constant improvement structure shall be implemented in this phase to drive RI" number to RPN = 25 level.

The manufacturing process control can be modified based on the results of FMEA. Also FMEA provides the analysis of critical factors and correlation of process variables with prod- uct results. Cause and effect (Fishbone) diagrams are used for this purpose.

Details FMEA (objectives, template, etc.) are presented in Table 3.

Several metrics are used to measure the result of the FMEA and to exit this phase: the RPN value and RPN Pareto Analysis, percent distribution of critical factors with different values of RPN (50, 100, 125, etc.), a list of problems with their prioritization (Criticality List), list of recommended corrective actions with the final RF" value, list of fail-safe solutions.

PROCESS CONTROL AND PROCESS CAPABILITY Process Capability Study. The major purpose of process capability study is to discover whether a process is in a steady state (state of statistical control), and if it is, whether the prod- uct will meet the quality requirements, or if the process has ability to hold product tolerances.

Control Charts are used in process capability study to :

1. Attain a state of statistical control (all group averages and ranges within control limits, there- fore, no assignable causes of variation present)

2. Determine process capability. After the process is in statistical control, the limits of process variation shall be determined

3. Monitor process

Control charts have an important role in product acceptance. A control chart is a run chart that has control limit lines at the top, bottom, and middle of the display. It is used to detect whether variation in data is due to the inevitable variations that occur under normal conditions or to a specific cause or abnormal conditions.

Table 3 FMEA Y' WHYS")

FMEA k a systematic and structured method (tool, technique) to analyze, identify and prioritize all (and each) potential failure modes, &~sess the results (effects) of failures on the systedproduct/process/customer, determine the potential causes, and idenlffy actions to minimize, prevent or eliminate of the potential failures which result in enhancing (manufacturing) reliability/quality of the product through the (continuous) control over failures before they reach the customer. FMEA idea k PREVENTION.

OBJECTMB

> UNDERSTAND AND EVALUATE POTENTIAL FAILURE MODES AND CAUSES > JDE" ACTIONS TO F,LIMINATE/REDUCE THE CHANCE OF POTENTIAL FAILURES > ENHANCING MANUFACTURING RELIABILITY /QUALITY OF THE PRODUCT THROUGH THE CONTROL OVER FAIL- URES > UNDERSTAND THE INTERACTION BETWEEN UNIT PROCESS INPUT VARIABLES AND THEIR RESULTING IMPACT ON THE PRODUCT > IDENTIFY AND GENERATE PROCESS CONTROLJIWTING (PLAN) > RETAIN (SHORTEN) PROCESS LEARNING >SYSTEMATICALLY DRIVE YIELD IMPROVEMENTS z ON LINE PROCESS MONITOR

OUANTITATIVEMEASUREMENT

> FMEA CLASSIFIES AND RANKS EACH POTENTIAL FAILURE MODE ACCORDPIG WITH RISK - RISK PRIORITY - E R 0 5 RPN IS A COMBINED EFFECT OF SEvERITY(l), PROBABILITY OF OCCURRlENCE (2), AND LIKELIHOOD OF THE

> CORRECTIVE ACTIONS ARE DEVELOPED AND PRIORITIZE BASED ON RPN 5 IMPLEMENT FAIGSAVE TECHNIQUES > RPN ORDER-(1-1000). RPN CRITICAL VALUE/THRESHOL~l0-125 (conditional)

PROBLEM DETECTION (3) - RPN = (1) x (2) x (3)

PROCESS FMEA TEMPLATE

Page of

FMEA DATE: FMEAID: FMEA Rev.:

Fabrication Procesr ID: Immediate (Output) Product KD: Approva1:Design:

Assermbly Process ID: End Item Product Identification: ApprovakMFG:

Process Flow Revision: Responsible Engineer: Approval: FMEA:

PartName/ Processstep/ Process Potential Potentla1 S Potential 0 Present D Risk Recommended Comments

Part Sequence Step Failure Effectsof E Causesof C Controk E Priority Actions & andor

Number Number Function Mode Failures V Failures C forCausesT Number Resp.Person Status

General recommendations for selecting control charts for variable and attribute data types are presented in Table 4 (Ref. 1).

Statistical Process Control (SPC). To attain a state of SPC control charts are constructed and process performance data are compared to statistical control limit. The performance data con- sists of groups of measurements (rational subgroups) selected in regular sequence of produc- tion while preserving the order.

Conventional statistical theory tools are used to define subgroup sizes. Usually it takes 25 sub- groups for variable data with four or five measurements in each subgroup. Subgroup sizes for attribute data depend on the specific type of chart and the non-conformity rate.

If both control charts (for sample averages and sample ranges) fall within the respective statisti- cal control limits, and a process is operating without assignable causes of variation it is con- cluded that the process is operating in statistical control.

In this case a calculation for process capability (and 6 Sigma) represents the (inherent) capabil- ity of the process.

Table 4

GUIDE TO CONTROL CHARTS SELECTION *)

-----------*- ----__I-

DATATYPE PARAMETERS TYPICAL ADVANTAGES DISADVANTAGES COMMENTS PLOTTED USE

VARIABLES

x and R/s Subgroup average and Mscbiwdominant A good window into Complex caleulstions, Select Subgroup size, range or standard processes the statistical variation slow response, indirect frequency and number deviation of a process relationship between

control b i b and tolerance

- X and R IndivMualand Where only one Quicker, easier to plot Not as sensid;;e as X

~ b s r o u P range observation per and explain. Compares and R chart for lot is available directly to tolerance. subgroups.

---------_1__̂11__-- -- CumSum Cumulative sum of High cost product or Fsster response to an Complex, hard to PRE-Control k faster

deviation of subgroup test where 0 5 Sigma to abrupt s W t in mean explain simpler averages from a reference value

2 Sigma sh€ft k common than X and R chart.

-----..-.-.-- ------ ---- __---- -----------v.-..

ATTRIBUTES

P Fraction nonconforming Only attribute data Data is usually easier to Attribute data less useful 'P Number nonconforming available or to monitor obtain than variable d a b than variable data. U C Number of nonconformities more than one characteristic than variable charts attribute data.

Nonconformities per unit quauty of complex units with Calcuktiom are easier Sample size bigger for

Ref (1): J.M.Juran, Quality Control Handbook, McGraw-Hill, pp. 24.13, 1988

In theory, a process capability analysis should not be made until the process is in statistical con- trol. However, in practice, a comparison of capability to tolerance limits is required (process performance analysis). The danger in delaying the comparison is that the assignable causes may never be eliminated from the process.

The process capability study and measuring should be performed for each operatiordwork sta- tion and for the entire process in Module, System and Software stages.

Module Operations. Variable measurements are preferable type of data for a process capabil- ity study. X and R Control Charts (average and range of variable data) are used. Attributes data should be used only where variable measurements are impractical. The 6 Sigma variation is compared to the (upper or lower) tolerance (specification) limits, and Performance Index (Cpk) is calculated.

In general, for effective manufacturing processes the 6 Sigma limits of the product output shall be well within the customer’s expectations. The Cpk > 1.33 value is recommended for critical parameters.

Note, if the process is centered at the nominal specification and follows a normal probability distribution, 99.73 percent of production will fall within +/- 3 Sigma of the nominal specifica- tion and only about 0.27 percent of the product are defective For centered processes with nor- mal distribution Cpk=l is equivalent to 2,700 defects per million rate, and Cpk=1.63 is equiva- lent to about 1 defect per million rate.

The Cpk > 2.5 should be considered as a goal (especially for critical parameters) for provision of Zero Defect concept.

Conventional statistical theory calculations are used to compare and translate the variation of 6 Sigma to percent defect rate.

System Operations and Software. Attribute data in a form of defect criteria - defect rates and DPMO - are used for a capability study for system assembly processes and software.

Attributes data require large sample size (since shifts in the process average are not revealed by attributes data). Conventional statistical theory tools are used to define subgroup sizes. Sam- ple sizes are functions of the specific type of chart and the non-conformity rate.

Typical process Control Charts for attribute data are: P Charts - Percent Defective, PM Charts - Number of defectives, C Charts - Number of defects in sample, U Charts - Number of De- fects per unit.

Several memcs are recommended to measure the result of the process capability study and to exit this phase:

1. Verification if each operation and the process are in SPC State.

2. Provision of required level of Cpk, DPMO and defect rate for each operation and the entire process. Defect Rate should be 63 per million.

3. Percent distribution of operations with different value of Cpk (more than 1.33, ... 2.0, etc.) for variable data, and defect criteria - DPMO and defect rate - value for attribute data.

4. Validation of recommendations of FMEA.

Design of Experiment. Experimental design techniques are used in special and complex cases when utilization standard techniques (FMEA, process capability analysis) do not provide the sufficient result, or sigtllficant improvement of process capability is required.

The use of experimental design techniques makes it possible to reduce process variation. Ex- perimental design techniques should identify parameters/factors which have a significant effect on a operatioarmess (so called control or critical factors), and to determine the optimum value of process variables which minimize the variation in a process while keeping a process mean on target.

PROCESS ACCEPTABILITY The objective of process acceptability activities is to demonstrate that the process is capable of producing zero process defects at sustained line output levels for period of time. All types of process defects are usuaUy counted. Process should demonstrate work performance at a zero defect level over a statistical valid number of Units. The sample size calculation is based on established confidence level and number of opportunities for defect (OFD) for each particular product line.

PROCESS MONITORING / MAINTENANCE

The objective of a process monitoring is to determine if a process is stable on continuous basis. The process and individual operations should continuously demonstrate if they can repeatedly produce the required level of product quality. To define if process is repeatedly producing prod- ucts to specification on continues basis all operations and the entire process are controlled pen- odically on daily, weekly, monthly and quarterly basis. 100% testing or testing on sample basis are usually used. A variety of sampling procedures are effectively applied. But it is important that sampling is done on a schedule basis and at time when the product is still traceable to specific streams of the process. All types of process de- fects are counted. Control charts are used to continually check the stability of the process, and to detect changes in the process before defective products are produced.

Cpk and defect rate levels are controlled. Cpk and defect rate requirements should be demon- strated on continuous basis (daily, weekly, monthly). Based on this a decision is done whether the operation or process should continue to run or to be adjusted.

ACHIEVING CERTIFICATION Process certification is achieved through compliance with the audit checklists, with specific cri- teria, and through demonstration that the process is capable of producing repeatedly required levels of Cpk, defect rate and yields. Certification indicates the following:

1. Fundamental organizational and functional structures of a manufacturing process and each operation are in place (FMEA, control system and feedback loop, documentation, training, etc.)

2. The entire process and each operation are mature and stable (under SPC).

3. Process Performance/Capability meets repeatedly quality goalshequirements on continuous basis for the entire process and individual operations

4. Independent audit demonstrates required level of product quality.

PROCESS IMPROVEMENT Provision of certification (as well as SPC) does not mean that the process has reached its opti- mum performance (or error levels). It is not an end point. Quite the contrary, it is the start of the improvement process and the beginning of the voyage to zero defect performance.

A process improvement should be driven by constant improvement of requirements (Cpk, de- fect rate levels) with the ultimate a long-range goal for process to produce zero defects.

CONCLUSION

The paper presents principles, processes, structure, guideline, measurements and criteria necessary to achieve and maintain manufacturing process certification.

Our approaches and experience in this area are discussed. The key element of our approach is to q u a e the results of work performed by the process, and to have measurements (such as RPN, Cpk, PPM, DPMO, etc.) of process performance/capability as a part of certification proc- ess.

The key difference between our and the standard industry approaches are:

- EMEA and SPC / Process Capability Study are required components of the certFficatfon process

- AU "Checklist" items must be met - there are no tradeoffs between items. No any scorehating is recommended.

- The Zero Defect demonstration is also required components of the certification process.

ACKNOWLEDGMENTS The authors would like to acknowledge our appreciation to the members of the Process Certifi- cation team D. Boyd, W. Carlson, S. Dunne and G. Swift.

REFERENCES 1. J.M.Juran, Juran's Quality Control Handbook, Editor in Chief, McGraw-Hill, 1988

2. L. Walsh, R. Wurster, R. J. Kimber, Quality Management Handbook, Marcel Dekker Inc., ASQC Quality Press, 1985